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On a simple network of about 10 computers setup with static IPs in the 192.168.1.x range, the ISP's DSL router occupies 192.168.1.1. All computers and the router physically connect to a simple network switch.

What I want to achieve is to put a mikrotik in the middle of the DSL and the computers, that is to plug the DSL router in one port of a mikrotik's 750GL and the switch to another port and have mikrotik bridge the traffic while at the same time appear as 192.168.1.1 for the computers in the network. It has to be done without changing the IP of the DSL router or any of the computers.

I know how to do it if the router and the computers are in a different network, but I don't know if it is possible in the same network and IP.

My final goal is to use the mikrotik as a transparent proxy to block access to certain sites. But this will be in a remote location where I won't be able to physically set it up, only prepare and sent the mikrotik. Additionally if something goes wrong at any point, I would like to be able to instruct someone to just take the plugs out to remove the mikrotik and connect them back to the switch in order to return to the previous working configuration.

I'd like to add that I tried to research this but I couldn't come up with the proper keywords or no such answer exists :)

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No, you can't have the same ip on two different interfaces, but using the mikrotik as a bridge/transparent firewall could help you accomplish your goal. you can set this under the bridge options. I have not worked with mikrotiks for a long time but if I remember correctly you can go to '/interface bridge settings' and set 'use-ip-firewall' to 'yes'. Probably you can also use bridge filters for this task.

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  • I'm marking your answer as accepted because it replies to the answer and gives a workaround. I missed the "hidden" ip firewall setting before, with that enabled one can mangle with packets in the IP level, not just the ethernet level that a normal bridge handles. Thanks.
    – user221931
    Jun 30 '14 at 11:59
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No, you cannot have two devices with the same IP address in one broadcast domain.

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  • Thanks for taking the time to reply. I know I can't have the same IP in the same network, but was hoping that there could be a way to separate the two networks in a physical (per port basis).
    – user221931
    Jun 10 '14 at 19:34
  • That was not what you asked :) The only way to do this is by creating two seperate networks using different IP-ranges. You cannot have the same IP-range on two different interfaces of the router.
    – Teun Vink
    Jun 10 '14 at 19:58
  • Well I was thinking it could be possible in the way one could bridge a 192.168.1.x<->192.168.2.x and then the opposite. But then I realized that's stupid too :)
    – user221931
    Jun 11 '14 at 1:58
  • Well, actually you can have two or more devices with static IP addresses with the same address. Just configure them that way! But it will result in conflicts and unpredictable behavior.
    – SPRBRN
    Jun 11 '14 at 15:12
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You can use different IP address and use Web proxy on Mikrotik. There is the option to redirect all http request through proxy. You can read more at http://wiki.mikrotik.com/wiki/Manual:IP/Proxy.

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